What significant discovery is associated with Schwann?

The significant discovery associated with Schwann is the assertion that all animals are made of cells. This concept is a crucial element of the cell theory, which he developed in collaboration with Matthias Schleiden in the 19th century. Schwann's work established that not only plants but also animals are composed of cells, which laid a foundation for understanding the biological organization of living organisms.

This discovery emphasized the idea that cells are the basic unit of life, which means that all tissues and organs in animals arise from cells. His research contributed significantly to the field of histology and provided vital insights into how life functions at a cellular level, establishing a commonality across different forms of life, both plant and animal.

The other options, while relevant to cell biology and its history, do not directly relate to Schwann’s specific contributions. The identification of a nucleus in all cells is linked to later discoveries by other scientists. The presence of a cell wall in plants is a botanical observation, and the theory of spontaneous generation pertains to a different area of biological thought that was eventually disproven.